In modern internal combustion engines, fuel is injected into a combustion chamber of the internal combustion engine by means of an injector. An injection is triggered therein by means of an electronic injection signal. Typical injection parameters, such as start of injection and injection duration, are specified by means of the injection signal. Depending on the operating point of the internal combustion engine, the injector is controlled differently in order to match the injection parameters to the current operating point. In this case the operating point of an internal combustion engine is typically determined by the load and the rotational speed of the internal combustion engine. In order to be able to perform an injection, modern control devices for internal combustion engines, referred to in professional circles as “engine control units” (ECUs), therefore include engine characteristic maps which map the operating point onto the injection parameters or, as the case may be, the injection signal. The engine characteristic map is in this case determined in advance on the test bench using a prototype of the respective internal combustion engine and subsequently stored in the ECU of an internal combustion engine of the respective series.
However, crucial metrics for optimal operation of an internal combustion engine are not the injection parameters, but combustion characteristic variables, such as, for example, the start of combustion, the combustion duration, or the combustion center of gravity. In each case, from the generation of the injection signal to the start of injection, and from the start of injection to the start of combustion, there elapses a period of time which, in internal combustion engines of the prior art, is already taken into account in the engine characteristic map as a constant delay. A disadvantage with this approach, however, is that combustion characteristic variables generally deviate from their setpoint values during operation. There are various causes for this:
Firstly, due to manufacturing tolerances of individual components and controllers of the internal combustion engine, more particularly of the injection system, the permanently stored engine characteristic map that was dimensioned for the prototype is, right from the outset, only approximately correct for a series-production internal combustion engine.
Secondly, the operating characteristics of the components, in particular injectors, change over the course of their useful life. The change in the operating characteristics of a component over the course of its lifetime is also referred to as “drift”.
The fact that commercially available fuel has different compositions constitutes a further problem. Thus, for example, fuel from different manufacturers, different gasoline stations or shipments can already exhibit significant differences in composition. This ultimately means that even given identical operating parameters of the internal combustion engine, the time gap between start of injection and start of combustion can vary for different tank fillings.